This invention relates to a propeller drive unit for motorboats and more particularly to a fuel efficient propulsor for motorboats.
It is the purpose of the marine propulsor to convert rotational shaft energy into a propulsive thrust. Ideally, this conversion is to be accomplished with the most efficient, vibration-free and inexpensive device. However, emphasis on achieving a particular goal such as efficiency, propulsor weight, mechanical simplicity or cavitation resistance may limit the designer in the type of propulsor configuration to be selected.
The most common and simplest propulsor that has been applied is the standard open propeller. A propeller operating in water experiences energy losses by two mechanisms. There are frictional losses as the blades pass through the fluid. An efficiency loss also occurs because energy is transferred to fluid by the blading and is lost in the slipstream.
Power losses associated with frictional effects on a rotating blade are approximately proportional to the cube of the blade-surface velocity and the wetted surface area of the propeller. To reduce frictional losses, the propeller should be small in diameter and have a minimum number of blades of small chord. The frictional losses are also reduced if the propulsor is designed to have a relatively high advance coefficient (ratio of ship speed to tip velocity of propeller blades). The blade-surface velocity will then be reduced to a value approaching the forward speed of the ship.
Reduction of frictional losses implies a small diameter propeller and a small mass flow rate of fluid through the propeller. The thrust produced is proportional to the product of the mass flow rate and the change in axial velocity of the fluid passing through the propeller. Therefore, producing a given value of thrust with a small mass flow rate requires large, changes in axial velocity and an excessively high slipstream velocity. A discharge jet with a high velocity results in low propulsive efficiency due to the large amount of kinetic energy that is dumped overboard in the jet. The high value of advance coefficient desired to reduce frictional losses requires the transfer of a large component of tangential velocity (swirl) to the fluid. For a small diameter propeller with a high advance coefficient, large kinetic energy losses are associated with both the axial and tangential components of slipstream velocity which decrease the efficiency of the propulsor.
It is evident that efforts to reduce frictional losses and kinetic energy losses in the slipstream dictate opposing design features, the highest efficiency achievable only by a proper balance between them.
The typical inboard or outboard motorboat has a high fuel consumption. The primary reason for this high fuel consumption is the low efficiency of the propeller and the fact that the roll or torque imbalance created by the propeller imparting angular momentum to the fluid must be reacted by the boat running at a condition that is not true and level. Ongoing efforts to provide fuel efficient power devices are quite important due to the high cost of fuel. It is thus desirable to reduce the high fuel consumption of such a system by reducing the swirl placed in the flow of the fluid and the torque imbalance on the boat.